In some applications, the total "out-leakage" of a large pressurized system must be accurately measured. The method used to measure this type of leakage with a mass spectrometer leak detector is called accumulation testing. The vessel to be tested is pressurized with a trace gas and placed in a sealed room with a leak detector and probe, or the instrument can be outside and the probe connected to the chamber through a port or opening. Any leakage of trace gas from the pressurized vessel will then be picked up by the detector. Since the concentration of trace gas in the room will be increasing with time, the output reading will also increase with time. The accumulation technique may be applied to the vessels of any size or configuration capable of being pressurized at greater than atmospheric pressure.
A conventional apparatus for accumulation testing is shown in FIG. 1. The free volume shown in FIG. 1 should be minimized where possible. This is recommended in order to reduce the time required to accumulate sufficient trace gas in the free volume for detection.
For the purpose of estimating the quantity of gas that accumulates the following relationship may be used: EQU P=Qt/V
where
Q=leak rate of gas into free volume leak rate specification. PA1 V=free volume. PA1 P=pressure change in free volume. PA1 t=elapsed time of gas leakage (Q) into free volume. PA1 1. Record the leak detector output versus time for the standard leak in a manner that will allow the gas to leak into the free volume of the system. PA1 2. After the calibration data has been acquired, close or remove standard leak from system, purge free volume of trace gas if necessary and pressurize the test vessel with trace gas. PA1 3. Record the leak detector output versus time for the item under test. When this data has been secured the total out-leakage may be compared and/or calculated from the standard calibration data. PA1 4. The unknown leakage may be calculated from the following expression: ##EQU1## where t.sub.1 =time for change in output when standard leak is valved into free volume PA1 LR.sub.1 =standard leak rate PA1 Output.sub.1 =net output for standard leak during t.sub.1 PA1 t.sub.2 =time for change in output when test port pressurized PA1 LR.sub.2 =unknown leak rate PA1 Output.sub.2 =net output as indicated during t.sub.2
The system can be calibrated in the following manner. Connect a standard leak in a manner that will allow the gas to leak into the free volume of the system.
A sampling probe used with a leak detector operates by continually extracting small amounts of gas from the volume being sampled. This extraction is performed via a vacuum applied within the probe. Because the differential pressure between the sampling probe and the lead detector needs to be maintained, the flow through the probe must be restricted. Thus, the sampling probe must be moved very slowly when attempting to locate leaks. Likewise, when attempting to detect gross leaks from a test item within a non-evacuated enclosure, it can take several hours for the reading on the leak detector to stabilize. Furthermore, the repeatability is not consistent.